This invention relates to a method and apparatus for making thermoplastic bags. More specifically, this invention relates to a method and apparatus for transferring and bundling thermoplastic bags of the type used to sack consumer goods in a grocery store, convenience store, drug store and the like.
Thermoplastic convenience bags have gradually gained acceptance by the retail industry and have completely replaced the utilization of paper bags in many market areas. A typical plastic bag includes a heat sealed bottom and integral carrying handles adjacent a top opening portion prompting some to descriptively refer to the bags as plastic tee-shirt bags.
In the past, tee shirt and other plastic bags have been manufactured by high speed processing machines wherein a roll of tubular thermoplastic stock is mounted upon one end of the machine and stacks or bundles of completed bags exit from an opposite end of the machine for packing. More specifically, the tubular plastic is unreeled by a pair of opposing drive or nip rolls which are coupled to an electric motor through a clutch and brake mechanism. The drive rolls are thereby suitable to be intermittently stopped and started to advance the plastic tubular stock at a predetermined rate and pattern of control. A cutting and sealing station is positioned downstream from the drive rolls and includes a first heat sealing strip, an intermediate, heated, cutting knife and a parallel, bottom heat, sealing strip. Actuation of the sealing and cutting mechanism is synchronously controlled with the advancement of the tubular stock so that the stock is stopped during a cutting and sealing operation, and the bag is sealed and cut and then advanced a predetermined length. The process is repeated in a rapid manner at a rate of approximately 120 to 150 bags per minute.
Downstream of the cutting and sealing station is a stack or bundle forming mechanism wherein a plurality of bags are stacked into a neat arrangement until a predetermined number is formed into a bundle for further transfer, processing and packing.
One previously known thermoplastic bag making machine, of the type generally described above, is illustrated in U.S. Pat. No. 3,580,142. In this patent disclosure, sheet material, which is flattened synthetic resin tubing, is transferred in specific lengths to a sealing and cutting station. Production of a single bag is referred to as a bag making cycle. After each cycle the completed bags are stacked to form a bundle. When the number of bags in a bundle reaches a specified total, the bag making operation is temporarily stopped or interrupted and the bundle of plastic bags is transferred by a transferring mechanism to an index conveyor where completed bundles are formed into a shingle array for further handling and packing.
In accordance with one previously known device, the bag making operation was stopped for one bag making cycle to permit operation of the transferring mechanism for a bundle. In another machine, such as the '142 patented structure, the duration of the stoppage was selected to be independent of one bag making cycle. In both instances, however, with previously known machines, bag production is temporarily stopped to prevent the thin bag material from becoming jammed during a bundle transfer operation. As a result of this stopping, to perform the transfer operation, production of the bag making operation was suspended which resulted in a reduction in production efficiency.
Although the production of thermoplastic bags in accordance with previously known techniques has achieved a degree of success and industry recognition. It has been found that production in accordance with conventional techniques is limited to approximately to 150 cycles per minute. Increasing the bag making cycle tends to increase a tendency for mis-alignment, wrinkling, and jamming of the relatively thin bags as well as increasing the wear on various moving components. In addition, it has been found that in operating machinery with a fixed bag production cycle, that operators have tended to remove damaged or jammed bags or material during the rapid bag making operation of the machine without stopping the bag making machine. Attempting to clear fouled material during full speed bag making can present a significant safety hazard.
Still further, conventional plastic bag making machinery, which runs at a constant speed, can present setup problems in the sense that the machine produces bags at such a rapid rate that it is difficult to visually perceive and verify the correctness of a single bag making operation.
The difficulties suggested in the proceeding are not intended to be exhaustive but rather are among many which tend to reduce the effectiveness and user satisfaction with prior methods and apparatus for transferring and bundling plastic bags. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that thin, plastic bag making methods and apparatus appearing in the past will admit to worthwhile improvement.